When you create a new genin you will be randomly assigned to a team in your village, with a B or higher rank shinobi as your sensei.

Teams are made up of three members, these will be your teammates and are the ideal way to spar with or complete missions. Your sensei is in charge of your training, but can also join you on missions, allowing you to take on tougher missions.

Your squad and sensei are also the ideal way to travel safer outside the village. Be sure to take this chance to bond with your teammates!

Additionally, an S-ranked shinobi can take on a pupil and become his or her mentor. This is roughly the same as the sensei-squad system, however this is purely optional. A mentor is able to focus more on a pupil, and can choose whoever he wants, regardless of rank, for whatever reason. However, the mentor can't have squad beside his pupil.

The list of squads can be found in a sticky in your village. So if you're new you should keep an eye on it to see in what squad you're put.